As shown in FIG. 10, among the holographic recording methods proposed heretofore is a holographic recording method in which either one of signal light 4 and reference light 6 branched out from a laser light source is made to cross the other in a recording layer 2A of a holographic recording medium 2 at a predetermined incidence angle θ, and the incidence angle θ is changed at a predetermined angular pitch Δθ so that information is recorded in an angle multiplex fashion as a hologram (for example, see Japanese Patent Laid-Open Publication No. 2003-178460).
By the way, in order to improve the recording density of the holographic recording medium 2 like this, it is important to “increase the thickness of the recording layer.” In general, the thicker the recording layer 2A of the holographic recording medium 2 is, the narrower the ranges of the incidence angles to satisfy the Bragg's condition becomes and the more the angular selectivity improves. Specifically, as shown in FIGS. 11(A) and 11(B), the relationship between the incidence angle of the reference light 6 and the diffraction efficiency, i.e., the angular selectivity varies with the thickness of the recording layer 2A. As compared to the medium having a thinner recording layer 2A shown in FIG. 11(B), the medium having a thicker recording layer 2A shown in FIG. 11(A) has narrower ranges of the incidence angles of the reference light 6 to satisfy the Bragg's condition, and has higher angular selectivity.
As above, the thickness of the recording layer 2A can be increased to improve the angular selectivity for the purpose of increased recording multiplicity and improved recording density. In recent years, demands for the holographic recording medium 2 to have higher recording capacities have been growing increasingly, and it is expected that the recording layer 2A will be thickened rapidly in the future.
Nevertheless, simply thickening the recording layer 2A has the problem that it is impossible to ensure reproduction compatibility between media that have recording layers 2A of different thicknesses (different angular selectivities) since the condition as to the incidence angle varies from one medium to another.